1. Field of the Art
This invention relates to a complex or composite optical element which basically has inherent optical functions as a prism, half mirror or reflector mirror, and more particularly to a lens-laminated composite optical element having a curved lens portion laminated on a plane surface of a vitreous optical element, either on a plane of incidence or a plane of emergence of light entrance of the vitreous optical element to provide complex or composite optical functions in an optical system.
2. Prior Art
Optical pickup devices are widely in use for reading and writing information on optical discs or similar information recording media. In this connection, currently both CD (compact disc) and DVD (digital versatile disc) are in use as information recording or storage media, and attempts have been made to develop and put in use a CD/DVD drive which is capable of reading CDs and writing and reading DVDs, for example, as described in Japanese Laid-Open Patent Application H10-27378. CDs and DVDs differ from each other not only in disc thickness but also in wavelength of laser beam, which is approximately at 780 nm in the case of CDs and approximately at 650 nm in the case of DVDs. Besides, CDs and DVDs require pickups which are different in NA (numerical aperture). Therefore, an optical pickup for a CD/DVD drive needs to be able to switch the laser light source between a CD reading laser light source and a DVD wring/reading laser light source. The laser light source can be switched, for example, by the use of a polarized prism. More particularly, the polarized prism is arranged to transmit a laser beam of 780 nm from a CD laser light source and to reflect a laser beam of 650 nm from a DVD laser light source. Thus, a light path from the polarized prism and a information recording medium is shared between a CD and a DVD for the purpose of providing an optical pickup which is downsized and compact in form.
In this connection, it often becomes necessary to use an optical lens like a spherical or aspherical lens in combination with the polarized prism for the purpose of correcting one or certain kinds of aberrations such as astigmatic aberration, spherical aberration and coma aberration which occur as the laser beam upon transmission through the polarizing prism. In various optical systems including optical pickups, a convex lens, a concave lens and an aspherical lens with a curved surface like cylindrical surface or a toroidal surface are used for converging a light flux to a light path or for shaping a light beam into a desired beam pattern.
In a case where a vitreous optical element like a polarized prism is used in combination with various optical lenses as mentioned above in an optical system, it becomes necessary to give independency to each one of the lenses to be incorporated into the optical system. However, in such a case, since individual optical elements are large in thickness, the optical system inevitably becomes large in size and weight. In this connection, it is also possible to form a lens surface integrally on a transparent vitreous substrate by injection molding a transparent synthetic resin. The integral molding of a plastic lens on a vitreous optical element makes it possible to downsize optical assembly units into a compact form. However, integral molding of a plastic lens has problems in that a plastic lens portion is large in linear expansion coefficient and inferior in resistance to environmental factors such as temperature and humidity. Alternatively, it is conceivable to add lens functions by injection molding a glass lens on a prism although glass injection molding is very costly and difficult to adopt.